The generation of leukotrienes (LTC4, LTD4, LTE4, and LTB4; 12-epi-LTB4 isomer) from human granulocytes by thiol-activated toxins (streptolysin O, alveolysin from Bacillus alvei, and theta toxin from Clostridium perfringens) is described. The release occurs under noncytolytic conditions. Although LTB4 is the major component after calcium ionophore stimulation, more LTC4 as compared with LTB4 is released with the toxins. The 5-lipoxygenase pathway of toxin-mediated activation can effectively be inhibited by caffeic acid, a lipoxygenase inhibitor. The toxins also induce the release of leukotriene-metabolizing enzymes such as gamma-glutamyltranspeptidase, which transfers LTC4 into LTD4, and dipeptidase, which metabolizes LTD4, into LTE4. Dipeptidase activity is more pronounced than the gamma-glutamyltranspeptidase activity but still does not reach the levels obtained when cells were triggered with opsonized zymosan.